Heterogeneous networks (HetNets or HTNs) are now being developed wherein cells of smaller size are embedded within the coverage area of larger macro cells and the small cells could even share the same carrier frequency with the umbrella macro cell, primarily to provide increased capacity in targeted areas of data traffic concentration. Such heterogeneous networks try to exploit the spatial distribution of users (and traffic) to efficiently increase the overall capacity of the wireless network. Those smaller-sized cells are typically referred to as pico cells or femto cells, and for purposes of the description herein will be collectively referred to as small cells. Such heterogeneous networks try to exploit the spatial variations in user (and traffic) distribution to efficiently increase the overall capacity of the wireless network.
In a co-channel network, an umbrella macro cell and the overlaid pico cell(s) share the same carrier frequency, the inter-cell interference between macro and pico cells becomes a challenging issue in HetNets. The inter-cell interference affects its coverage area and the effective offload capability of a small cell. The inter-cell interference may cause of the UE handover failures and mobility performance degradation.
Such deployments present some specific interference scenarios for which enhanced inter-cell interference coordination (eICIC) techniques would prove beneficial.
In one scenario, the small cells are pico cells, which are open to users of the macro cellular network. In order to ensure that such pico cells carry a useful share of the total traffic load, user equipments (UEs) may be programmed to associate preferentially with the pico cells rather than the macro cells, for example by biasing the received signal power of the Common Reference Symbol (CRS), a quantity that may be referred to as reference signal received power (RSRP), such that UEs that are not too close to a pico cell will associate with the pico cell. Despite the association, UEs in pico range extension area or near the edge of a pico cell's coverage area will suffer strong interference from one or more macro cells. In order to alleviate such interference, some subframes may be configured as “almost blank” in the macro cell. An “almost blank” subframe is a subframe with reduced transmit power (e.g., reduced from a maximum transmit power) and/or a reduced activity subframe (e.g., contains only control information as compared to a fully loaded subframe). Legacy UEs (also called terminals) expect to find the reference signals for measurements but are unaware of the configuration of these special subframes.